Study of microstructure and corrosion behavior of nano-Al2O3 coating layers on TiO2 substrate via polymeric method and microwave combustion

The study describes the successful development of a TiO2 ceramic substrate with a protective nano-Al2O3 coating using two different coating techniques: microwave combustion and polymeric methods. The coated ceramics demonstrate enhanced corrosion resistance compared to the uncoated substrate. The optimal TiO2 substrate was prepared by firing it at 1000 °C. This was done to give the desired physical properties of the TiO2 substrate for the coating procedures. Nano-Al2O3 powder was coated onto the surface of the TiO2 substrates. The TiO2 substrates with the Al2O3 coating were then calcined (heat-treated) at 800 and 1000 °C. The structures, morphology, phase composition, apparent porosity, bulk density, and compressive strength of the substrate and coated substrate were characterized. Upon firing at 1000 °C, it was discovered that the two phases of TiO2—rutile and anatase—combine in the substrate. Once the substrate has been coated with nano Al2O3 at 1000 °C, the anatase is transferred into rutile. When compared to the substrate, the coated substrate resulted in a decrease in porosity and an increase in strength. The efficiency of the ceramic metal nanoparticles Al2O3 as a good coating material to protect the TiO2 substrates against the effect of the corrosive medium 0.5 M solution of H2SO4 was measured by two methods: potentio-dynamic polarization (PDP) and the electrochemical impedance spectroscopy (EIS). The results indicated that the corrosion rate was decreased after the substrate coated with alumina from (67.71 to 16.30 C.R. mm/year) and the percentage of the inhibition efficiency recorded a high value reaching (78.56%). The surface morphology and composition after electrochemical measurements are investigated using SEM and EDX analysis. After conducting the corrosion tests and all the characterization, the results indicated that the coated TiO2 substrate prepared by the polymeric method at 800 °C displayed the best physical, mechanical, and corrosion-resistant behavior.

The polymeric (Pechini) method is based on the polymerization of metallic citrate with ethylene glycol.An aqueous solution containing hydrocarboxylic acid, such as citric acid, was utilized to chelate cations.An organic ester is created when a polyalcohol, like ethylene glycol, is added.Heating-induced polymerization creates a homogenous resin with evenly dispersed metal ions throughout the organic matrix.
In a beaker with 20 ml of water, a specific amount of AlCl 3 .6H 2 O is dissolved.Citric acid and ethylene glycol are added to this water volume in a 60:40 mass percent ratio.
The calculated amount of citric acid was dissolved in 10ml of millipore water followed by constant stirring at 60 to 70 °C.A clear and homogenous sol solution of citric acid was prepared.Once added, the salt, AlCl 3 .6H 2 O (12.11 g), is continuously stirred until completely dissolved.Finally, the calculated dose of ethylene glycol was added with continuous stirring 22 .The first part of the freshly prepared solution is used for the coating procedure.The other part is subjected to drying at 100 °C, followed by firing for an hour at different temperatures, 800 °C and 1000 °C, to obtain the nano-alumina powder to undergo the powder investigations.
The prepared coating solution is deposed upon the titania substrate surface by dipping the TiO 2 substrates in the freshly prepared coating solution for two hours; the coating solution containing the substrates is suctioned.The coated substrate samples are then taken out of the sol solution, dried in the air at 100 °C, and then fired for an hour at two different temperatures: 800 °C and 1000 °C.The coated substrates' deposition, drying, and calcination procedures took place twice to obtain a uniform covered alumina coating layer on the titania substrate's surface.

Preparation TiO 2 substrates coated by alumina prepared via microwave combustion method
The microwave combustion method includes preceding salt (AlCl 3 .6H 2 O) added to the urea fuel to create the combustion solution.Millipore water was employed as a solvent during the preparation of samples.To create a uniform solution, the compounds were first dispersed individually in millipore water and blended for about an hour at room temperature.The inorganic components in the precursors functioned as the oxidizers in the combustion process, while urea served as the fuel.There was a 2:1 M ratio between the fuel (urea) and the oxidizer (AlCl 3 .6H 2 O) (F/O).A particular amount of AlCl 3 .6H 2 O is dissolved in 20 ml of water in a beaker and then mixed with the calculated weight of urea 23 .The first part of the freshly prepared solution is used for the coating procedure.In contrast, the other part is subjected to calcination at 800 °C and 1000 °C after exposure to dried air at 100 °C to obtain the nano-Al 2 O 3 powder prepared via microwave for powder investigations.
The deposition procedure of the prepared alumina coating solution on the titania substrates surfaces took place by dipping the TiO 2 substrates in the prepared alumina homogenous solutions (pre-solution) in silica crucible and placed inside a microwave oven and exposed to 1000 W of electricity at 300 °C for two minutes at a frequency of 2.45 GHz.The coated TiO 2 substrates in solution are removed from the microwave and put under suction for two hours.The microwave synthesized coated titania substrates by alumina layer are then removed from the solution and dried in the air for 1 h, first at 100 °C and then fired at two different temperatures: 800 °C and 1000 °C.The coated substrates' dipping, microwave irradiation step, drying, and calcination procedures were taken twice to obtain uniform covered alumina coating layers on the titania substrate's surface.The two coating methods are illustrated in Table 1 and graphically represented in Fig. 1.

Characterization
X-ray diffraction (XRD) was carried out to examine phase composition using monochromatic Cu K radiation (D 500, Siemens, Mannheim, Germany).Moreover, Fourier transform infrared spectra were examined by a Japanese Jasco-300E FTIR spectrophotometer.The synthesized samples were crushed, and the fine powders were combined in a 1:100 ratio with KBr.The particle size was examined using a transmission electron microscope (TEM) model JEOL JEM-2100 outfitted with an x-ray energy-dispersive spectroscopy system and a high-angle angular dark-field detector.The structure and morphology of the materials were ascertained using Scanning Electron Microscopy (SEM) observations and FEI, QUANTA FEG, 250.According to ASTM C373-88, apparent porosity and bulk density were calculated using the Archimedes principle 25 .A LLOYD device, Model LR 10K, was utilized to study the compressive strength of samples using the specific method in 26 The detection of the corrosion process took place in 0.5M solution of H 2 SO 4 for the prepared discs of uncoated TiO 2 substrates and for coated TiO 2 substrates by nano-Al 2 O 3 layers, which were prepared by two different The deposition, drying, and calcination procedures took place twice to obtain a uniform alumina covering layer techniques: microwave and polymeric methods at two different temperatures; 800 and 1000 °C.The detection of the corrosion process was carried out by using a potentiostat/galvanostat with electrochemical impedance spectroscopy Autolab PGSTAT302N.The electrochemical cell was configured with three electrodes: Ag/AgCl electrode as the reference electrode, platinum electrode as the counter electrode, and the coated and uncoated TiO 2 substrates acting as the working electrode.Open circuit potential (OCP), potentiodynamic polarization (PDP), and electrochemical impedance spectroscopy (EIS) are the electrochemical methods that were used to measure corrosion behavior.The potentials of the corrosion process are recorded concerning the reference electrode (Ag/AgCl).

Physical properties of TiO 2 substrate samples
The physical properties are indicated in terms of bulk density and apparent porosity.The effect of different sintering temperatures at 900, 1000, and 1100 °C on the apparent porosity and bulk density of the TiO 2 substrate sample are clearly shown in Fig. 2. As demonstrated in Fig. 2, the apparent porosity reduced, and bulk density increased by increasing the sintering temperature.The apparent porosities at 900, 1000, and 1100 °C are 48.98,30.76, and 10.20%, respectively.The bulk densities are 1.18, 2.55 and 3.69 g/cm 3 at 900, 1000 and 1100 °C.In this study, it is important to prepare titania substrate structure in a porous form to allow penetration of nano-coating solution through the pores of the substrate to facilitate adhesion to its body followed by complete covering of its surface by nano-alumina layers to increase the corrosive resistance of the coated titania substrates to a high extent 27 .Accordingly, the TiO 2 substrate samples fired at 1000 °C were selected as the optimal substrates to be applied for the coating procedures.are both TiO 2 phases, are combined.The primary peak is at 25.3°, which corresponds to the crystal plane (101) for the TiO 2 anatase phase and the crystal plane (110) for the TiO 2 rutile phase at 27.4°2 8 .Generally, the synthesis process used in preparing the titania powder precursors significantly impacts the phase transition from anatase to rutile, varying from 850 °C in the sol-gel approach to 1100 °C in the solid-state reaction 29 .

IR analysis of TiO 2 substrate sample
Figure 4 shows the FTIR spectra of the substrate TiO 2 samples sintered at 1000 °C according to XRD results.Signals between 400 and 1000 cm −1 correspond to Ti-O-Ti vibration 30 .Abroad absorption band between 450 and 800 cm −1 regions is ascribed to the vibration assigned to absorption of the Ti-O-Ti linkages in TiO 2 particles 30 .
For the pure TiO 2 , the peak at 420 cm −1 is related to the presence of anatase titania.According to the standard spectra of TiO 2 , the peak at 499 and 850 cm −1 may be or was attributed to the vibration of the Ti-O bond in the TiO 2 (rutile titania) lattice.The small IR absorption band at 870 cm −1 is attributed to the Ti-O-Ti stretching vibrations 31 .

SEM of TiO 2 substrate sample
The morphology of the TiO 2 substrate is seen in Fig. 5a after calcination of the sample at 1000 °C.The SEM images demonstrate the TiO 2 particles appeared in spherical and rod shapes.Rod-like anatase particles were covered by  small spherical particles of rutile 32 .The particle sizes of rod TiO 2 shapes ranged between 4 to 15 µm.The spherical particles of TiO 2 ranged from 100nm to 2µm.EDS spectra spectrum shown in Fig. 5b showed that only Ti and O elements were detectable, and no other elements were identified for the samples studied.The uniformity of the synthesized TiO 2 consists of well-interconnected crystallite.The percentage of the surface porosity of the prepared titania substrate was calculated to be about (30%).Thus, some pinholes can be observed in the sample surface.
TEM analysis of TiO 2 substrate sample TEM of powder TiO 2 material fired at 1000 °C is shown in Fig. 6.It is indicated by the well crystallization of TiO 2 particles with octahedral geometry; as shown in Fig. 6, the geometry of the rutile form of titanium dioxide is twisted hexagonal with a size ranging between 0.10 to 0.17µm.) for the alumina powder prepared by polymeric method (Poly).On the contrary, alumina's main corundum phase (α-Al 2 O 3 ) is obtained using the microwave combustion method (MW).Thus, the microwave method is more favored in increasing the crystallinity degree of the prepared alumina, which is indicated by a decrease in the γ-Al 2 O 3 phase peak intensity 23 .www.nature.com/scientificreports/

IR analysis of Al 2 O 3 powder
The FTIR spectra of prepared alumina powders calcined at 800 and 1000 °C and prepared by polymeric method (Poly) and microwave method (MW), respectively, are shown in Fig. 8.It was observed that alumina powders calcined at 800 °C showed an appearance of γ-Al 2 O 3 .The calcined alumina powder prepared by polymeric method showed sharp bands corresponding to the stretching vibration of Al-O appearing at 720 and 513 cm -1 while they appeared at 812, 740, and 436 cm -1 for the alumina calcined powder prepared by microwave method 16 .Bands for α-Al 2 O 3 appear at 639, 576, and 437 cm -1 for powders calcined at 1000 °C and prepared by microwave method.Meanwhile, bands around 1084 and 1095 cm -1 are assigned to the symmetric bending of Al-O for calcined powders at 800 and 1000 °C, respectively 16 .From these results, it was observed that the band intensity of the alpha alumina at 1000 °C is high and appeared at 437 cm -1 compared with the calcined samples at 800 °C that were prepared by both polymeric and microwave methods; these results are confirmed by the XRD results.
TEM analysis of Al 2 O 3 powder TEM images of alumina powder prepared by the polymeric method at different temperatures, 800 and 1000, are shown in Fig. 9a, b, respectively.The alumina prepared by microwave method is seen in Fig. 9c, d after firing at 800 and 1000 °C, respectively.It was observed that the prepared alumina powders by the microwave combustion method are more crystalline than those prepared by the polymeric method.Alumina particles synthesized by microwave technique are presented in a nanosphere shaped at 800 °C and changed into nanoplate forms by raising the temperature to 1000 °C, while the alumina particles prepared via polymeric method at 800 °C still retained the shape of the amorphous polymeric structure that included some nanosphere grains compared to microwave method.
These findings are consistent with other studies that show a rise in calcination temperature is accompanied by a sequence of transformations, including-γ- 24 .All alumina phases that originated at low temperatures changed into α -Al 2 O 3 at high temperatures.Low activation energies are required for transformations from one phase to another, whereas transformations proceed through nucleation, growth, and increased temperature 17,33,34 .The interaction of microwaves with the reactants at the molecular level, where this electromagnetic energy is transferred and transformed to heat by rapid kinetics through the motion of the molecules, can be used to explain why microwave heating causes the acceleration of the crystallinity and formation of alpha alumina at 1000 °C.Thus, the microwave combustion method causes the development of nanoparticles within a short period, the development of nanoparticles, early phase formation, and various morphologies [35][36][37] .

Phase composition of coated TiO 2 substrate
The XRD patterns of the TiO 2 substrate fired at 1000 °C and coated with Al 2 O 3 by utilizing polymeric and microwave combustion methods, then firing at 800 and 1000 °C, are shown in Fig. 10.Through the XRD of TiO 2 -Al 2 O 3 and its comparison with pure titanium substrate, which is seen in Fig. 3, an enhancement in the crystallization of  TiO 2 was observed by increasing the firing temperature.By firing the samples at 800 °C, the main anatase phase is indicated with small amounts of rutile phase.In comparison with pure substrate, the rutile peak intensities decrease.This can be attributed to the reaction between some γ-Al 2 O 3 particles and rutile particles, leading to a small amount of aluminum titanate phase.However, the presence of the anatase phase at 800 °C as the main phase is due to some γ-Al 2 O 3 particles surrounding the anatase phase.It prevents the growth of anatase crystals and their transformation to rutile at this temperature 38 .However, upon increasing the temperature to 1000 °C, the anatase phase disappears for the coated samples prepared by polymeric and microwave is observed.The  rutile phase is the only phase obtained at this firing temperature compared with the pure substrate, as presented in Fig. 3.This can be attributed to the transformation of γ-Al 2 O 3 to α-Al 2 0 3 at 1000 °C, leading to the stability of the anatase phase.This work is conceded with Young Cheol Ryu 39 observed that the metal cations diffusing from the substrate into the TiO 2 layer might retard the Anatase-Rutile phase transformation of TiO 2 .The suppressing effect on the Anatase-Rutile transformation of TiO 2 by mixed cations seems much stronger than that of single cations.In addition, it was observed that the Anatase -Rutile transformation in the TiO 2 substrate deposited by α-alumina might proceed more easily, and the TiO 2 substrate deposited by α-alumina has a higher rutile fraction 39 .It was reported that the phase transformation of TiO 2 depends on various parameters such as the initial particle size, impurity (doping) concentration, starting phase, and reaction atmosphere 39,40 .This work illustrates that the phase transformation of TiO 2 from anatase to rutile is influenced by the metal ions (Al 3+ ) diffused from the coated layer into TiO 2 substrate as well as calcination temperature.Eskelinen et al. 41 , who studied the effects of heat treatment on the surface composition of TiO 2 thin film in TiO 2 -phlogopite and in TiO 2 -muscovite system by X-ray photoelectron spectroscopy technique, claimed that the surface composition was dependent on the calcination temperature and the substrate components diffusing through the TiO 2 film 39,41 .

IR analysis of coated TiO 2 substrate
The IR patterns of the TiO 2 substrate after being coated with Al 2 O 3 using the polymeric method and microwave combustion method fired at 800 and 1000 °C are seen in Fig. 11a, b, respectively.In comparison, the IR analysis of the pure TiO 2 substrate that fired at 1000 °C with TiO 2 substrate coated with alumina, it was found that the band at 420 cm −1 is corresponding to anatase after firing at 800 °C as seen in Fig. 11a, while the bands at 499 and 850 cm −1 that showed in Fig. 11b after firing at 1000 °C refers to rutile phase.Bands related to Ti-O bond vibrations are found in the 493-579 cm −1 and 594-639 cm −1 ranges, as seen in Fig. 11a 42 .Bands caused by the stretching vibrations of the Al-O bonds of the octahedrally coordinated Al were seen in the 500-750 cm −1 range, whereas bands caused by the vibrations of the Al-O bond in AlO 4 units are presented in the 750-900 cm −1 range, as seen in Fig. 11a 42

Physical properties of coated TiO 2 substrate
Figure 12 depicts the physical characteristics of bulk density (BD) and apparent porosity (AP) of coated TiO 2 substrate samples coated with alumina, created using polymeric and microwave techniques, and fired at 800 and 1000 °C temperatures.The apparent porosity increases and the bulk density decreases as the firing temperature rises, as was seen.Overall, all of the samples show pore characteristics.Porosity may result from several different factors, according to Yang 43 , including (1) gas formation during the sintering process and the subsequent expansion, entrapment, or escape of those gases, (2) shrinkage related to the sintering reaction (i.e., products with a higher specific volume than the starting materials), or (3) residual initial porosity of the powder due to partial sintering.When comparing titania-coated substrates with alumina made using the polymeric approach to those made using the microwave method, it was discovered that the microwave method produced coated substrates with greater porosity than the polymeric method.This is attributed to the absorption of Al 2 O 3 into TiO 2 samples using the polymeric method, which is higher than the microwave method and is indicated by the decrease in porosity.Another reason for this phenomenon is the higher crystallinity of the alumina produced by microwave as opposed to the polymeric method enhances the formation of some secondary aluminum titanate phase (Al 2 TiO 5 ), which is difficult to sinter and has a lower density of 3.7 g/cm 3 compared to that of titania at 4.23 g/cm 344 .Due to the presence of an Al 2 TiO 5 phase that is less to be detected in XRD (Fig. 10), high-temperature firing could not improve the densification percentage 44  metastable phase known to include more different doping elements Ti 4+ than α-alumina does 45 , which affects the grain boundaries (GB) diffusion properties, may be the cause of the decrease in porosity of the coated samples at 800 °C.γ → α Phase transition is described as a type of nucleation and growth transformation that affects the porosity quantities in the samples, according to S. Lartigue-Korinek et al. 45 .

SEM of the uncoated and coated TiO 2 substrates with nano-Al 2 O 3
The SEM images of TiO 2 coated substrate with Al 2 O 3 synthesized via polymeric and microwave techniques and fired at 800 °C at different magnifications are shown in Fig. 13.It was observed that the surface of the coated TiO 2 substrate by alumina prepared via microwave method has larger pinholes than the coated substrate with alumina prepared by polymeric method.In addition, the EDX analysis shows more absorption of Al ion for coated samples prepared by the polymeric method than the microwave method.This is attributed to the Al  absorption to a high extent.This is due to the crystallinity of prepared alumina by microwave (α-alumina) being higher than that prepared by polymeric method (γ-alumina), as shown previously in TEM result Fig. 9, as the metal ions with large atomic radii diffused less readily than those with smaller atomic radii 46 .
The SEM images of TiO 2 substrates uncoated and coated with nano-Al 2 O 3 synthesized via polymeric technique and fired at 800 °C after they were exposed to 0.5 M solution of H 2 SO 4 are shown in Fig. 14a, b, respectively.Figure 14a illustrates the shape of the uncoated TiO 2 substrate after the PDP was carried out in the aggressive medium 0.5 M H 2 SO 4 , It was clear that the surface of the sample contains many holes and roughness, and these characteristics are due to the effect of the solution.
EDX analysis and its data tabulated in Table 2 found that the weight percentage of titanium in the substrate before the PDP was executed was about (54%).In comparison, it became (44%) after the PDP was achieved.This is due to the presence of sulfur element from the (SO 4 2− ) group, as its weight percentage is recorded at about 3%; thus, it penetrated the surface of the substrate, which led to a reduction in the weight percentage of titanium.Also, the effect of the penetration of sulfur ions to the surface of the substrate is demonstrated by increasing the porosity of the uncoated TiO 2 substrate from 30% before PDP to 33% after PDP, as was established in Table 3, which means that the surface of the sample had been damaged and has many pores.
Figure 14b represents the coated TiO 2 substrate with nano-Al 2 O 3 prepared by the polymeric method and calcined at 800 °C after being exposed to the aggressive medium 0.5 M H 2 SO 4 solution and tested by the PDP method.It can be seen that the surface of the substrate no longer has pinholes, and the percentage of the surface porosity was calculated; it was found to be about 23% (Table 3); this means that the presence of the nano-Al 2 O 3 helped to coat the surface well, which stimulated the reduction of pores.In addition, it can be noticed that the surface of the sample became free from cracks and gaps without scratches and also smoother.This result proves the alumina's effect in protecting the sample's surface 47 .The presence of a few pores in the range of nano-scale on the sample's surface is a good factor in forming an oxygen barrier as a protective layer for the substrate in the corrosive medium 48 .The data obtained from EDX analysis showed that the percentage weight of the oxygen is about 70%, and this result explains the formation of an oxygen barrier, and the high percentage weight of oxygen may be related to the reduction of sulfate ions 49 .www.nature.com/scientificreports/Compressive strength for the uncoated and coated TiO 2 substrate Upon calcination at 800 °C, a few selected coated samples are examined for compressive strength and contrasted with uncoated substrate samples.Unlike microwave-coated samples and uncoated substrates, the TiO 2 -coated substrates with alumina via polymeric approach exhibit the highest strength among the remaining samples, with a maximum strength of about 65 MPa.This can be attributed to some of the Al 2 O 3 nanoparticles entering the titania substrate's pores and filling them while the rest of the nano-Al 2 O 3 particles well covered the titania substrate surface, as discussed previously in the SEM results; this behavior led to an increase in the strength of the TiO 2 coated substrates compared to uncoated TiO 2 substrates.The polymeric approach Al 2 O 3 nanoparticles' tiny particle size with appropriate distribution produces the maximum reinforcing effect compared to the uncoated TiO 2 substrate samples 50 .They are absorbed by the titania substrates, which reduces the titania substrate porosity compared to the alumina prepared via the microwave process, which has the opposite effect on strength values.Additionally, it has been found that treating the uncoated titania substrates with sulfuric acid decreased their compressive strength values to approximately 28 MPa, while treating the coated titania substrates with alumina prepared via microwave and polymeric method with sulfuric acid decreased their compressive strength value from 55 and 65 to be 45 and 52 MPa, respectively.As presented in Fig. 15.

Corrosion test estimation
Open circuit potential measurements.The open circuit potential of the uncoated and coated TiO 2 substrate with alumina (Al 2 O 3 ), which was prepared by polymeric (poly) and microwave (MW) methods at 800 °C, was measured in the aggressive medium 0.5M solution of H 2 SO 4 .The experimental results of the OCP were presented in Fig. 16 as three curves that have analogous shapes with an approximate difference: (a) the substrate without coating, (b) the substrate coated with alumina by the (MW) method, and (c) the substrate coated with alumina by the (poly) method.It can be seen from Fig. 16 that the OCP was shifted in a positive direction, from about − 0.382 V for the uncoated TiO 2 substrate to about − 0.363 V for the coated TiO 2 substrate with alumina, which was prepared by (MW) method and − 0.321 V for the coated TiO 2 substrate with alumina which prepared by (poly), method against silver/silver chloride.This result was attributed to forming a protective coating layer on the surface of the TiO 2 substrate.As well as this result also confirms that the polymeric method for coating was better than the microwave method, and this result agrees with the above discussion.Potentio-dynamic polarization method.The PDP experiment is one of the successful methods used to determine the corrosion rate of the substrate exposed to aggressive media like sulfuric acid, nitric acid, hydrochloric  acid, and sodium chloride and the inhibition efficiency of the inhibitor or the coating which used to hinder the corrosion process 51 .The coating process is considered one of the important methods to increase the efficiency of some materials by changing some of their physical properties to protect the metals and their alloys from the undesirable effects of the corrosion process 52 .Among the materials used in this field are ceramic materials such as Al 2 O 3 , ZrO 2 , and TiO 2 53 .According to the data and the results of the PDP experiments for the TiO 2 substrates uncoated and coated with alumina (Al 2 O 3 ), after exposing them to a 0.5 M solution of sulfuric acid as an aggressive medium, it was found that the corrosion rate of the uncoated substrate was decreased from the value (67.71 to 16.30 and 29.75 mm/year), after it was coated with Al 2 O 3 by polymeric (poly), and microwave (MW), methods and calcined at 800 °C, respectively.By comparing the corrosion rate values of the two coating methods, it can be noticed that the polymeric method was better than the microwave method.This result may be attributed to the shape of the phase formed by alumina on the substrate after it was coated, as was reported in previous studies 54,55 .The analysis and characterization of the TiO 2 substrate explained that, after the substrate  was coated with alumina, which was prepared by the microwave method at a temperature of 800 °C, the coating layer was formed in the alpha form of aluminum oxide crystallized in the corundum structure, which has some properties such as low surface area and is almost non-porous 56 .The coating layer of alumina prepared by the polymeric method at the same temperature was formed in the gamma alumina phase, which has some excellent properties, such as a large surface area with some porosity, which resulted in decreasing the corrosion rate of the substrate-coated by polymeric method more than the substrate coated by microwave method at 800 °C, as shown in Table 4 57 .Equation (1) was used to deduce the percentage of the inhibition efficiency of the substrate after it was coated with nano-Al 2 O 3 to prevent the corrosion process, and all the parameters are established in Tables 4 and 5.
where i o corr and i corr are the corrosion current density of the uncoated and coated substrate, respectively.Good values of the inhibition efficiency (56% and 78%) of the working electrode (TiO 2 disc), after it was coated with alumina, which was prepared at 800 °C by microwave and polymeric methods and tested in the corrosive medium (0.5 M H 2 SO 4 ), can be interpreted by reducing the active area on the substrate due to the formation of mix film of Al 2 O 3 , TiO 2 phase 58 and TiAl phase; which it has excellent mechanical properties and corrosion resistance at temperatures (over 600 °C) 59 .The appearance of smooth polarization curves in Fig. 17a, b and only one peak at the corrosion potential is evidence that the formed film was electrochemically inactive 60 .This result agrees with the previous characterization of the surface.It was clear from the results reported in Table 4 that the value of the corrosion potential (E corr ), of the uncoated substrate (TiO 2 disc), recorded − 409.13 mV, shifts to more noble value in a positive direction after coating with Al 2 O 3 to reach − 289.73 mV at (800 °C), for the sample coated with alumina by polymeric method, while the corrosion potential (E corr ), recorded − 382 mV, when the sample coated with Al 2 O 3 by microwave method at (800 °C); this result verifies that the use of the polymeric method (Poly), for coating was better than the microwave method (MW).Also, this result confirms the formation of a good coating layer of the nano-composite formed of alumina and titanium (Al 2 O 3 TiO 2 phase and TiAl phase) that exhibits good resistance against the corrosion process 61 .As mentioned by M. Sabzi et al., the potentiodynamic polarization diagram for galvanized steel in seawater environment electrolyte shows that the galvanized layer's resistance to corrosion is greater than that of the steel underlying.Moreover, the impedance resistance of galvanized steel decreased as the surrounding temperature rose 62 .
Through Table 4, it can also be noted that the corrosion current density ( i o corr ), was decreased from the value of 712.62 μA cm −2 of the uncoated TiO 2 substrate to reach a value of about 313.15 and 152.79 μA cm −2 of the coated substrate by the nano alumina that, synthesized by microwave and polymeric methods, respectively.This result indicates that the formed film increases the ability of the substrate to resist the corrosion process 2 .
Electrochemical impedance spectroscopy method.The EIS diagrams of the uncoated and coated substrate (TiO 2 disc), after tested in 0.5 M H 2 SO 4 solution, are depicted in Figs. 18, 19, 20 and 21.It can be seen from Fig. 18a, b that the Nyquist plots deviated from the ideal semicircle, meaning that it appeared as a depressed semicircle.This phenomenon, the effeteness of the frequency dispersion, has been attributed to forming a passive film with microporous, roughness and irregularity of the surface of the substrate, and random distribution of the active sites 63,64 .
Increasing the diameter size of the semicircle of the substrate (TiO 2 disc) after it was coated with alumina (Al 2 O 3 ) pointed to the formation of a good passive layer on the surface of the sample attributed to the presence of alumina, which exhibited good stability in various media; as it allows improving the corrosion resistance of the sample surface 65 .In addition, the comparison between the diameter size of the two capacitive loops of the  substrate (TiO 2 disc), after it was coated with alumina, which was prepared by microwave and polymeric methods Fig. 18a, b, respectively, showed that; the diameter size of the capacitive loop Fig. 18b, was larger more than that of Fig. 18a, this result agrees with the above discussion that the polymeric method, was better for coating than the microwave method.According to previous studies, the large diameter of the semicircle of the sample also means more protection against the effects of the corrosion process 66 .
The percentage of the inhibition efficiency (IE%) of the coating material (Al 2 O 3 ) was calculated using.R ct according to the following equation: Abbreviation R o ct and R ct refers to the charge transfer resistance of coated and uncoated samples after the EIS test.It was cleared by reading the results reported in Table 5 that the charge transfer resistance.R ct increases after the substrate (TiO 2 disc) is coated with nano-(Al 2 O 3 ); this result indicates that the corrosion rate is reduced due to the formation of a protective film of the coating materials on the surface of the sample.
The equivalent circuit used for the analysis of EIS data was [R(QR)(QR)], shown in Fig. 19, where (R s ) is the solution resistance, (R ct ) the charge transfers resistance and (R f ) the film resistance.Due to the presence of the phenomenon frequency dispersion; the constant phase element (CPE), was used in the equivalent circuit instead of the double-layer capacitance (C dl ), to be more suitable for the results of the impedance data 67 .
The constant phase element (CPE), was determined from the following equation: ( According to the above equation, the constant phase element (CPE) consists of a constant Y o , j is an imaginary number equal (− 1) 1/2 , ω is the angular frequency in rad/s, and a component n is the exponent of CPE expresses phase shift where n is between (− 1) and (+ 1), i.e. (− 1 ≤ n ≤ + 1).So, if a component n = 0, the constant phase element acts as a resistor, while n = − 1, the CPE appears as an inductor, and if n = + 1, the constant phase element performs as a capacitor 68 .Shifting of the phase angle or the component n to more positive value after the substrate (TiO 2 disc), coated with nano-Al 2 O 3 , by microwave method from (0.443 to 0.525 at 800 °C) and after the substrate coated by polymeric method from (0.443 to 0.983 at 800 °C), this result refers to that, the irregularity of the sample surface was decreased 69 , as be reported in Table 5, and can also be seen in Fig. 20a, b.
Also, it can be observed from Table 5 that the value of n is less than 0.5 for the substrate without coating; this result means that the corrosion rate was controlled by the diffusion process, while after the substrate was coated,

Conclusion
This work successfully created a nano-Al 2 O 3 coated layer on TiO 2 substrates using polymeric and microwave techniques to extend the ceramic field applications.The following results were observed: 1.A temperature of 1000 °C was found to be ideal for preparing the TiO 2 substrate samples.2. Phase composition of TiO 2 substrate samples revealed that anatase and rutile phases were formed at 1000 °C.
3. The nano-Al 2 O 3 coating layer was created using polymeric and microwave techniques.According to the findings, after firing at 800°C, the polymeric method produced more nano γ-Al 2 O 3 than the microwave method.4. Raising the temperature to 1000 °C increases the formation of α-Al 2 O 3 production using the microwave method.5.The formation of anatase at 800 °C and rutile at 1000 °C are caused by the Al 2 O 3 deposition on the TiO 2 substrate 6.At 800 °C as opposed to 1000 °C, the coated TiO 2 substrates with Al 2 O 3 showed superior mechanical and physical properties.7. Good compaction was obtained using the polymeric method rather than the microwave method for preparing the Al 2 O 3 coated layer on TiO 2 substrates at 800 °C.8. Following treatment with an aggressive sulfuric acid medium, coated TiO 2 substrates with nano-Al 2 O 3 had higher compressive strength than uncoated TiO 2 substrates.9.According to the results of the corrosion tests, the nano alumina (Al 2 O 3 ) can be used as a good coating material to protect the TiO 2 substrate against the effect of the corrosive medium 0.5 M solution of H 2 SO 4 .

Figure 2 .
Figure 2. The effect of sintering temperatures on the bulk density and apparent porosity of TiO 2 substrate.

Figure 5 .
Figure 5. SEM image and corresponding EDS spectra of TiO 2 substrate sintered at 1000 °C.

Figure 7 .
Figure 7. XRD patterns of Al 2 O 3 powder prepared by polymeric (Poly) and microwave methods (MW) at 800 °C and 1000 °C.

Figure 8 .
Figure 8. FTIR spectra of Al 2 O 3 powder prepared by polymeric (Poly) and microwave methods (MW) at 800 °C and 1000 °C.

Figure 9 .
Figure 9. TEM images of Al 2 O 3 powder prepared by polymeric method at temperature (a) 800 °C, (b) 1000 °C and alumina prepared by microwave method at temperature (c) 800 °C, (d) 1000 °C.

Figure 10 .
Figure 10.XRD patterns of TiO 2 substrate coated with Al 2 O 3 powder using polymeric (Poly) and microwave (MW) methods fired at 800 and 1000 °C.

Figure 11 .
Figure 11.FTIR patterns of TiO 2 substrate coated with Al 2 O 3 powder by using polymeric (Poly) and microwave (MW) methods and fired at (a) 800 and (b) 1000 °C.

Figure 12 .
Figure 12.Physical properties of the coated TiO 2 substrate with Al 2 O 3 synthesized by polymeric (Poly) & microwave (MW) techniques and fired at 800 and 1000 °C.

Figure 13 .
Figure 13.SEM of coated TiO 2 substrates with Al 2 O 3 synthesized via (a) polymeric and (b) microwave techniques and fired at 800 °C at different magnifications.

Figure 14 .
Figure 14.SEM image and EDX analysis after the PDP process of (a) the uncoated TiO 2 substrate in 0.5 M H 2 SO 4 solution and (b) the coated TiO 2 substrate with nano-Al 2 O 3 prepared by polymeric method in 0.5 M H 2 SO 4 solution.

Figure 15 .
Figure 15.Compressive strength values for uncoated and coated titania substrates before and after exposure to sulfuric acid treatment.

Figure 16 .
Figure16.Open circuit potential of TiO 2 substrate in 0.5 M H 2 SO 4 solution (a) the substrate without coating (b) the substrate after coated by alumina prepared by (MW), method (c) the substrate after coated by alumina prepared by (poly), method examined in 0.5M solution of H 2 SO 4 at 25 °C.

Figure 17 .
Figure 17.Tafel plots of uncoated and coated TiO 2 substrate by the nano-Al 2 O 3 after PDP process (a) coating by microwave method (MW) (b) coating by polymeric method (Poly), examined in 0.5 M solution of H 2 SO 4 at 25 °C.

Figure 18 .
Figure 18.Nyquist plots (a) TiO 2 substrates uncoated and coated with nano-Al 2 O 3 prepared by microwave method (MW), (b) TiO 2 substrate uncoated and coated with nano-Al 2 O 3 prepared by polymeric method (Poly) tested in 0.5 M H 2 SO 4 at 25 °C.

Figure 19 .
Figure 19.The modal of the equivalent circuit used to fit the impedance data.

Figure 20 .
Figure 20.Bode phase of the impedance data (a) TiO 2 substrates uncoated and coated with (Al 2 O 3 ), formed by microwave method (MW), (b) TiO 2 substrates uncoated and coated with (Al 2 O 3 ), formed by polymeric method (Poly), tested in 0.5 M solution of H 2 SO 4 at 25 °C.

Figure 21 .
Figure 21.Bode modulus of the impedance data (a) TiO 2 substrates uncoated and coated with (Al 2 O 3 ), formed by microwave method (MW), (b) TiO 2 substrates uncoated and coated with (Al 2 O 3 ), formed by polymeric method (Poly) tested in 0.5 M solution of H 2 SO 4 at 25 °C.

Table 1 .
The sequence of coating process parameters.
6The freshly prepared sol.used for dipping the Ti substrate is put in the microwave for 2 min, as in the microwave method 7The coating sol.Containing Ti substrates are suctioned for 2 h 8 After that, the coated substrate was removed from the sol.And dried at 100 °C for 24h 9 Then, it fired for an hour at 800 and 1000 °C 10

Table 2 .
Element composition of uncoated and coated TiO 2 substrate by alumina prepared by (Poly) method at 800 °C before and after the PDP process in 0.5 M H 2 SO 4 solution.

Table 3 .
Effect of the porosity percentage on the corrosion rate of the TiO 2 substrate uncoated and coated by alumina by polymeric (Poly) and microwave (MW) methods after tested in 0.5 M H 2 SO 4 solution.

Table 4 .
The parameters of the PDP process of uncoated and coated TiO 2 substrates with Al 2 O 3 prepared by microwave method (MW) and polymeric method (Poly) in 0.5 M solution of H 2 SO 4 at 25 °C.

Table 5 .
The electrochemical impedance data of the TiO 2 substrate uncoated and coated by nano-Al 2 O 3prepared by microwave method (MW) and by polymeric method (Poly) tested in 0.5 M H 2 SO 4 at 25 °C.